1. Field of the Invention
The present invention relates to an interpolation apparatus in an encoder.
2. Description of the Related Art
There is an apparatus called encoder, which produces a sin θ signal and a cos θ signal whose phases are mutually different by 90° in response to movement of an object to be measured. The encoder then binarizes these produced signals based on a reference value, and counts changes of each binarized signal so as to detect the distance of the object to be measured.
As a method of improving an encoder's resolution, various interpolation methods have been proposed such as, for example, a resistance division method and a phase modulation method. Among these methods, the resistance division method lowers the interpolation precision when the number of divisions increases. The phase modulation method deteriorates the real time performance since time is calculated from a zero cross to a zero cross of a phase modulation wave.
As the first method of solving such a problem, there is a method shown in Jpn. Pat. Appln. KOKAI Publication No. 2-186221, for example. An interpolation apparatus in an encoder shown in the Jpn. Pat. Appln. KOKAI Publication No. 2-186221 incorporates means for digitizing a sine component and a cosine component, means for calculating polar coordinates of a detection signal from the digitized sine and cosine components, and means for dividing one cycle of the detection signal, using the polar coordinates, so as to perform the interpolation. The interpolation apparatus in the encoder converts each of two signals of the sine component and the cosine component into polar coordinates information, and divides one cycle in sine wave based on the angle information so as to perform the interpolation.
Also, as the second method, there is a method shown in Jpn. Pat. Appln. KOKAI Publication No. 2-38814. The method in the Jpn. Pat. Appln. KOKAI Publication No. 2-38814 provides calculating phase angle data corresponding to a sine component and a cosine component in advance, storing the phase angle data in a memory device, and reading out the sine component and the cosine component as address signals from the memory device.